The present invention relates in general to directional microphones and, in particular to a new and useful directional microphone of the interference and reflector type having a single elongated tube with apertures therethrough with a transducer at one side thereof and a reflector connected to the opposite side thereof.
The principal drawback of prior art constructions for directional microphones of the interference type is that at higher frequencies, the transmission factor is worse than at all the other frequencies. The deterioration at high frequencies is particularly due to the absorbtion of these frequencies on the inside of the tube wall, mainly at locations where acoustic frictional resistances are provided. In addition, the load of the air column in the tube on the transducer diaphragm increases the transmission factor in the mid-frequency range, whereby the impression of an unsatisfactory transmission of the high frequencies is amplified.
To remedy these drawbacks, the attempt has been made to replace the single tube by a tube bundle where the tubes have different lengths. With a careful acoustic tuning of the individual tubes of the bundle, an improvement is in fact obtained, however, such a bundle, of course, is less easy to handle than a single tube, aside from the rather difficult tuning of the individual tubes of the bundle.
Microphones, such as interference microphones, with a sharply beamed directional characteristic, include directional microphones which are equipped with a reflector that mostly corresponds in shape to a parabolic mirror. The microphone of this type is placed at the focus of the parabolic reflector. With such microphones, requirements must be met making a reduction to practice this type of microphone difficult.
To obtain an effective concentration of low frequencies, a reflector of large and unhandy dimensions is needed, since in principle, the diameter of the reflector should at least be equal to the wavelength of the lower-limit frequency to be transmitted. Because such microphones are used primarily outdoors, their construction must be extraordinarily stable to stand transportation and weather conditions. Insufficient stability substantially impairs the directional effect and the sensitivity of these devices.